Pressurized water nuclear reactors comprise components of large size such as steam generators, which have a partly cylindrical outer housing of large diameter and very great height with a wall of medium or great thickness.
This outer wall is generally constructed by the butt welding of cylindrical shells to each other or to an annular assembly flange of a tube plate or a frustoconical shell.
In any case, there is effected an assembly of two annular parts disposed in the axial extension of each other and having their axial ends abutted against each other.
The electron beam welding makes it possible to produce without a filler metal, welded joints of very high metallurgical quality with production times which are considerably reduced as compared with conventional techniques, such as arc welding.
However, one of the difficulties in carrying out the electron beam welding results from the necessity to create a relatively high vacuum around the region of the connection of the parts onto which the electron beam is directed.
In the case of parts of modest size, it is possible to effect the welding inside a fluidtight enclosure enclosing the electron gun in which the parts are introduced and placed in their position of assembly before the enclosure is closed in a fluidtight manner and evacuated.
It will be clear that, in the case of components of very large size such as steam generators, the cost of the installations comprising fluidtight enclosures of very large size and the cost of the operation may become prohibitive.
Consequently, it is envisaged to create a vacuum in a limited volume around the region in which the electron beam welding is effected. However, in the case of the butt welding of annular or tubular parts, the construction of fluidtight enclosures which permit surrounding the welding region presents certain difficulties. In particular, it is necessary to dispose means for constituting a fluidtight enclosure both on the exterior and on the interior of the annular or tubular parts which have joint surfaces abutted against one another.
The electron beam welding is effected by the fusion of the metal of the parts to be welded in a very narrow region along the joint surface of the parts. This fusion is obtained by displacing the electron beam which is directed in the direction of the joint surface of the parts so as to scan the whole of this surface. In the case of annular or tubular parts, the gun producing the electron beam is usually disposed on the outside of the parts and the beam is directed from the exterior toward the interior. It is necessary to dispose a welding heel constituted by a metal strip or member placed in contact with the surfaces of the parts in the region of their joint, on the side opposed to the surfaces onto which the beam is first of all directed. In the case of annular or tubular parts, the welding heel must be placed in contact with the cylindrical inner surface of the parts along .the joint region. It has been found to be extremely difficult to provide and place in position a welding heel which is in perfect contact with the inner surface of the parts throughout their internal periphery.
Further, the welding heel, which undergoes a slight fusion during the electron beam welding of the parts, is fixed after welding on the surface of the parts and throughout their periphery. The separation of the welding heel from the welded parts may present certain difficulties owing to the fact that the heel is fixed against the parts throughout its periphery by welding, and possibly owing to the effect of stresses exerted between the parts and the heel.
Therefore difficulties are usually encountered both when placing in position the heel inside the parts and when separating it from the parts after welding.
Further, the geometrical and metallurgical quality of the assembly resulting from the welding of the two annular or tubular parts partly depends on the positioning of the parts with respect to each other before the welding operation proper.
Joint surfaces of generally frustoconical shape are machined on each of the parts to be joined, these surfaces being superposed before welding.
In the case of parts of very large diameter, and more generally in the case where the ratio between the diameter of the parts and their thickness is very large, the positioning of the annular or tubular parts in perfectly coaxial positions may present certain difficulties if means are not provided for guiding and positioning the parts to ensure perfect superpositioning of their joint surfaces.
Heretofore, no device was known for disposing inside annular or tubular parts for effecting their connection by electron beam welding which permitted peforming all the functions mentioned hereinbefore which are required if electron beam welding of high quality is to be effected under economical conditions.